(1) Field of the Invention
The present invention relates to a CATV (cable television) tuner apparatus which receives a voice signal delivered on a CATV channel.
(2) Description of the Related Art
A CATV system is usually configured by using a coaxial cable to carry a CATV signal from central equipment. For example, FIG. 3 shows a configuration of the CATV system of this type.
In the CATV system of FIG. 3, central equipment 20 and a plurality of bidirectional trunk amplifiers 10 through 19 are linked together by a coaxial cable having a relatively large diameter. This trunk cable runs through the CATV system distributing the CATV signal to each neighborhood. Each of the trunk amplifiers 10-19 includes feeder lines that branch off the trunk typically use a smaller diameter cable to carry the signal into the neighborhoods passing each subscriber""s home. In FIG. 3, xe2x80x9cAxe2x80x9d denotes a subscriber connected to the trunk amplifier 19, xe2x80x9cBxe2x80x9d denotes another subscriber connected to the trunk amplifier 13, and xe2x80x9cCxe2x80x9d denotes another subscriber connected to the trunk amplifier 16.
CATV signals from the central equipment 20 are usually distributed on the trunk cable at a certain level per channel. It is necessary that the signal level delivered to the home is higher than the desired lowest level per channel. In order to maintain the signal level, the trunk amplifiers 10-19 are spaced along the lines at certain distances that are less than a permissible maximum distance.
The trunk amplifiers 10-19 serve to branch the CATV signal transmitted by the central equipment 20 downstream to subscribers and to deliver it to the individual subscribers. Further, the trunk amplifiers 10-19 serve to insert the subscriber information into the signal delivered upstream to the central equipment 20. Hence, the two-way communications between the subscribers and the central equipment 20 are carried out with the CATV system.
In the CATV system of FIG. 3, video information is delivered from the central equipment 20 to the trunk amplifiers 10, 14, 17, 18 and 19 on the trunk cable along the downstream path, in this order. For example, the. trunk amplifier 19 branches the video signal from the central equipment 20 and supplies it to the subscriber A connected to the trunk amplifier 19. Further, the trunk amplifier 19 inserts subscriber information from the subscriber A into the signal that is delivered upstream to the central equipment 20 in the opposite direction.
FIG. 2 shows a CATV tuner apparatus according to the related art. Specifically, the CATV tuner apparatus of FIG. 2 is applicable to an input module of a CATV tuner. The CATV tuner is located at each subscriber""s home and connected through a coaxial cable to one of the plurality of trunk amplifiers within the CATV system of FIG. 3.
As shown in FIG. 2, the CATV tuner apparatus according to the related art includes a tracking filter 1, a band-pass filter (BPF) 2, an automatic gain control (AGC) 3, an amplifier (AMP) 4, a band-pass filter (BPF) 5, a frequency synthesizer (FS) 6, and a local oscillator (OSC) 7. In FIG. 2, reference numeral 8 denotes an input module of a CATV tuner located at each subscriber""s home. The local oscillator 7 generates a predetermined oscillation frequency which is supplied to the frequency synthesizer 6.
In the CATV tuner apparatus of FIG. 2, the tracking filter 1 serves to perform, through a filtering process, a channel selection in which a specific one of multiple CATV channels delivered on the coaxial cable is selected, based on an input signal Vin (for example, a video signal) received at an input terminal of the apparatus. The band-pass filter 2 eliminates the out-of-band noise components from the received signal from the tracking filter 1 and passes only the in-band signal components through the band-pass filter 2.
In the CATV tuner apparatus of FIG. 2, the automatic gain control 3 detects the level of the signal received from the band-pass filter 2, and either increases or decreases the amplitude of the signal before passing the signal to the amplifier 4. Each of the tracking filter 1, the band-pass filter 2 and the automatic gain control 3 will attenuate the level of the received input signal Vin. The amplifier 4 amplifies the level of the signal received from the automatic gain control 3. The band-pass filter 5 eliminates the out-of-band noise components from the received signal from the amplifier 4 and passes only the in-band signal components through the band-pass filter 5. The frequency synthesizer 6 produces an intermediate-frequency (IF) signal through a multiplication (or a division) of a frequency of the signal, received from the band-pass filter 5, by the oscillation frequency supplied by the local oscillator 7. In the CATV tuner apparatus of FIG. 2, the IF signal output by the frequency synthesizer 6 is supplied to the output terminal.
Referring back to FIG. 3, in the above-described CATV system, when the CATV signal is delivered from the central equipment 20 to the CATV tuner apparatus (or the subscriber""s home) via some of the trunk amplifiers 10-19, or vice versa, the signal level delivered to the CATV tuner apparatus is maintained at an appropriate level by the bidirectional trunk amplifiers. For example, in a case of the signal transmission between the central equipment 20 and the subscriber A, the signal level delivered to the CATV tuner apparatus (connected to the trunk amplifier 19) is maintained at the appropriate level by the transit trunk amplifiers 10, 14, 17, 18 and 19. In a case of the signal transmission between the central equipment 20 and the subscriber B, the signal level delivered to the CATV tuner apparatus (connected to the trunk amplifier 13) is maintained at the appropriate level by the transit trunk amplifiers 10, 11, 12 and 13.
However, in a case of the transmission of a voice signal between the subscriber A and the subscriber B over the CATV system of FIG. 3, the signal level delivered to the CATV tuner apparatus is not properly maintained at the appropriate level. The signal transmission from the subscriber A to the trunk amplifier 10 in the upstream direction may be performed such that the signal level delivered to the trunk amplifier 10 is properly maintained by the transit trunk amplifiers. However, the subsequent transmission of the same signal from the trunk amplifier 10 to the subscriber B has to be performed in the downstream direction, and the signal level delivered to the subscriber B is not properly maintained by the transit trunk amplifiers. In the above case, the signal level delivered to the CATV tuner apparatus on the receiver side becomes too low.
In the case of the voice signal transmission between the subscribers over the CATV system, such as that shown in FIG. 3, the following problems will arise with the CATV tuner apparatus of FIG. 2 because the level of the input signal Vin received at the input terminal is quite low.
1. The level of the received input signal is significantly low, and the tracking filter 1, the BPF 2 and the AGC 3 of the CATV tuner apparatus of FIG. 2 act to attenuate the level of the received signal and add a certain noise to the received input signal. The carrier-to-noise ratio (CNR) of the received signal will deteriorate considerably. In a case of the reception of an analog voice signal, the quality of the reproduced signal deteriorates. In a case of the reception of a digital voice signal, the bit error rate of the received signal is raised.
2. In a case of the reception of a digital audio signal from another node over the CATV system, the bit error rate of the received signal will be significantly raised. It is difficult that the CATV tuner apparatus of FIG. 2 produces a high quality signal from the received signal.
3. When the voice signal is received by using one of a large number of CATV channels (for example, 700 channels) on the coaxial cable, a CNR degradation of the received signal caused-by interchannel influences is more severe. It is difficult that the CATV tuner apparatus of FIG. 2 provides a stable signal reception capability.
An object of the present invention is to provide an improved CATV tuner apparatus in which the above-described problems are eliminated.
Another object of the present invention is to provide a CATV tuner apparatus which effectively minimizes the bit error rate of the received input signal and produces a high quality IF signal from the received input signal even when the level of the voice signal delivered between the subscribers over the CATV system is significantly low.
The above-mentioned objects of the present invention are achieved by a CATV tuner apparatus which receives an input signal at an input terminal and produces an intermediate frequency signal at an output terminal from the input signal, the apparatus including: a first low-noise amplifier which amplifies a level of the input signal received at the input terminal, the first low-noise amplifier having an adequately small noise figure to allow the received input signal to be amplified with no substantial degradation of a carrier-to-noise ratio of the input signal; an automatic gain control which detects the level of the signal amplified by the first low-noise amplifier and adjusts an amplitude of the signal before passing the signal through the automatic gain control; a band-pass filter which eliminates out-of-band noise components from the signal received from the automatic gain control and passes only in-band signal components through the band-pass filter; a second low-noise amplifier which amplifies a level of the signal received from the band-pass filter; and a frequency synthesizer which produces the intermediate frequency signal at the output terminal through a multiplication of a frequency of the signal, amplified by the second low-noise amplifier, by an oscillation frequency.
In the CATV tuner apparatus of the present invention, the use of the first low-noise amplifier at the input terminal and the second low-noise amplifier at the output of the band-pass filter enables a minimum of noise to be added to the received input signal, and allows the CATV tuner apparatus of the present invention to produce a high-quality intermediate frequency (IF) signal at the output terminal without substantial carrier-to-noise ratio (CNR) degradation. In a case of the reception of a digital audio signal from another node over the CATV system, the CATV tuner apparatus of the present invention is effective in minimizing the bit error rate of the received signal and producing a high quality IF signal. Further, in a case of a CNR degradation of the received signal caused by interchannel influences, the CATV tuner apparatus of the present invention is effective in providing a stable signal reception capability.